Polymer laminate possessing an intermediate water vapor transmission barrier layer

ABSTRACT

An opaque barrier layer which comprises: 
     (a) at least one thermoplastic polymer matrix substrate layer within which is located a strata of voids; 
     positioned at least substantially within at least a substantial number of said voids is at least one spherical void-initiating particle which is phase distinct and incompatible with the matrix material, the void space occupied by said particle being substantially less than the volume of said void, with one generally cross-sectional dimension of said particle at least approximating a corresponding cross-sectional dimension of said void; the population of voids in said substrate and the thickness of said substrate being such as to cause a degree of opacity of less than 70% light transmission; and, 
     (b) at least one thermoplastic water vapor transmission barrier layer affixed to said thermoplastic substrate layer.

BACKGROUND OF THE INVENTION

This invention relates to polymer films; and in particular, to laminatedpolymer films possessing water vapor transmission barrier capability.

U.S. Pat. No. 4,377,616, the contents of which are incorporated byreference herein, discloses an opaque biaxially oriented polymer filmstructure comprising a thermoplastic polymer matrix core layerpossessing numerous voids, a substantial number of which contain atleast one spherical void-initiating particle, and transparent skinlayers adhering to the surfaces of the core layer. The unique structureof the core layer imparts a much higher degree of opacity, possibly dueto the effects of light scattering, than that possible by the use ofopacifying pigment alone.

There is no suggestion, however, in U.S. Pat. No. 4,377,616 ofincorporating the film structure disclosed therein in a laminatepossessing a water vapor transmission barrier layer.

SUMMARY OF THE INVENTION

In accordance with the subject invention, an opaque biaxially orientedpolymer film laminate of lustrous satin appearance and possessing awater vapor transmission barrier layer is provided which comprises:

(a) at least one thermoplastic polymer matrix substrate layer withinwhich is located a strata of voids;

positioned at least substantially within at least a substantial numberof said voids is at least one spherical void-initiating particle whichis phase distinct and incompatible with the matrix material, the voidspace occupied by said particle being substantially less than the volumeof said void, with one generally cross-sectional dimension of saidparticle at least approximating a corresponding cross-sectionaldimension of said void; the population of voids in said substrate andthe thickness of said substrate being such as to cause a degree ofopacity of less than 70% light transmission; and,

(b) at least one thermoplastic water vapor transmission barrier layeraffixed to said thermoplastic substrate layer.

The process for preparing the foregoing polymer laminate comprises:

mixing a first thermoplastic polymeric material with a second materialincompatible with said first material to form a substrate mixture, saidsecond material being of a higher melting point or having a higher glasstransition temperature than said first material;

heating the substrate mixture to a temperature at least above themelting point of said first material;

dispersing said second material uniformly throughout the molten firstmaterial in the form of microspheres;

extruding the substrate mixture in the form of a substrate layer;

affixing at least one thermoplastic water vapor transmission barrierlayer to said substrate layer; and,

biaxially orienting the resulting laminate film structure at atemperature and to an extent to form opacifying voids in said substratelayer so as to optimize the degree of opacity, enhance the physicalcharacteristics and impart a lustrous satin appearance to said filmstructure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In preparing the thermoplastic polymer substrate layer (a) of thelaminate of the present invention, it is preferred that the averagediameter of the void-initiating particles be from about 0.1 to about 10microns. These particles should be spherical in shape so as to initiatea void of unusual regularity and orientation in a stratifiedrelationship throughout the thermoplastic polymer maxtrix material afterbiaxial orientation of the system. This does not mean that every void isthe same size. It means that, generally speaking, each void tends to beof like shape even though it may vary in its dimensions from those ofother voids because all of the voids are initiated by a sphericalparticle. Ideally, the voids assume a shape defined by two opposed andedge-contacting concave disks.

Optimum characteristics of opacity and satin-like appearance areobtained when the two average major void dimensions are greater thanabout 30 microns.

The void-initiating particle material should be incompatible with thesubstrate material, at least at the temperature of biaxial orientation.

The substrate has been described above as being a thermoplastic polymermatrix material within which is located a strata of voids. From this itis to be understood that the voids create the matrix configuration. Theterm "strata" is intended to convey the understanding that there are alarge number of voids creating the matrix and the voids themselves areoriented so that the two major dimensions are aligned in correspondencewith the direction of orientation of the polymeric film structure. Aftereach void has been formed through the initiation of one or morespherical particles, the particle(s) may contribute little else to thesystem. This is because its refractive index can be close enough to thatof the matrix material that is makes no contribution to opacity. Whenthis is the case, the opacity is principally a function of the lightscattering affect which occurs because of the existence of the voids inthe system. The opacity of the system can be somewhat enhanced by theinclusion therein of an opacifying pigment dispersed throughout. Aparticularly preferred proportion of pigment in this layer can be fromabout 1 to about 3% by weight of the core. The pigment material ispresent in such a particle size and shape that is does not, at least inany material sense, contribute any void initiation by itself. Theoptional presence of the opacifying pigment contributes perhaps 3 to 8%to the degree of opacity of the system.

A typical void of the substrate is defined as having major dimensions Xand Y and minor dimension Z, where dimension X is aligned with machinedirection orientation, dimension Y is aligned with transverse directionorientation and dimension X approximately corresponds to thecross-sectional dimension of the spherical particle which initiated thevoid.

It is a necessary part of the present invention that orientationconditions be such that the X and Y dimensions of the voids of thesubstrate be major dimensions in comparison to the Z dimension. Thus,while the Z dimension generally approximates the cross-sectionaldimension of the spherical particle initiating the void, the X and Ydimensions must be significantly greater. In addition, the orientationconditions must be such that the general integrity of the voids ismaintained. By this, it is meant that during the orientation whichproduces the X and Y dimensions, that is, either by simultaneous orsequential machine direction and transverse direction stretching, thetemperature conditions must be such as to permit these major dimensionsto form without any destruction of the voids in any of its dimensions.The voids are particularly vulnerable to destruction during sequentialorientation if the stretching temperature is too low. Even insimultaneous orientation if the temperature is too low, the stretchingforces will tend to cause internal shredding and void splitting. Thisleads to a complete loss of control over the integrity of the individualclosed voids, and the consequent integrity of the matrix polymer. Thus,one skilled in the art, following the present general guidelines canorient at a temperature and to a degree which will yield X and Ydimensions approaching a maximum without causing at least anysubstantial splitting, shredding or overall lack of void and matrixintegrity.

By way of illustration, room temperature biaxial orientation of apolypropylene matrix layer (a) containing nylon spheres of the size andamount contemplated herein will not produce the claimed structure.Either void splitting will occur or voids of insignificant size willresult. Polypropylene must be oriented at a temperature which happens tobe significantly higher than its glass transition temperature. Thetemperature conditions must permit X and to be at least severalmultiples of the Z dimension without void splitting at least to anysignificant degree. If this is accomplished, optimum physicalcharacteristics, including low water vapor transmission rates and a highdegree of light scattering, are obtained without void splitting or filmfibrillating.

As indicated above, the matrix polymer and the void initiating particlemust be incompatible and this term is used in the sense that thematerials are two distinct phases. The spherical void initiatingparticles constitute a dispersed phase throughout the lower meltingpolymer which polymer will, ultimately, upon orientation, become a voidfilled matrix with the spherical particles positioned somewhere in thevoids.

As a result of the biaxial orientation of the film structure describedtherein, in addition to opacifying the substrate layer of the structure,the orientation improves other physical properties of the compositelayers such as, flex-cracking resistance, Elmendorff tear strength,elongation, tensile strength, impact strength and cold strengthproperties. In addition to this, an extraordinary, rich looking,lustrous satin appearance is imparted to substrate layer (a). Thisappearance is not seen in the absence of water vapor transmissionbarrier layer (b) or if barrier layer (b) is too thin or otherwisesubstantially reveals the surface imperfections of substrate layer (a).

It is believed that because of the comparative sphericity of thevoid-initiating particles, the voids are closed cells. This means thatthere is virtually no path open from one side of the substrate to theother through which liquid or gas can traverse.

The void-initiating particles can be organic or inorganic so long asthey are spherical in shape and within the preferred particle size rangeso as to initiate the formation of a void, which in a significant numberof cases, has a lens-like shape, that is, a lens of the biconvex shape.When a polymeric material is contemplated as the void-initiatingparticle, it can be a polymer which is co-melted with the polymer of thematrix or substrate. In this case, it is necessary for it to have asufficiently higher melting point than the core polymer and beincompatible and capable of assuming a dispersed phase of smallspherical particles as the temperature of the co-melted mix is reduced.It is also contemplated that the void initiating particles can bepreformed and then uniformly dispersed into a melt of, e.g.,polypropylene. This has the advantage of not having to subject thematrix polymer layer (b) to the temperature of a much higher meltingpolymer. In this manner, any thermal degradation of the matrix polymeris avoided.

It is believed that because of the number, the shape and the orientationstrata-wise of matrix voids, a significantly enhanced light scatteringeffect is obtained. This effect is further enhanced or magnified by theuse of the water vapor transmission barrier layer (b) described below.

It is important that a particular thickness relationship exist betweenthe thickness dimension or the substrate layer (a) and the thicknessdimension of water vapor transmission barrer layer (b). It is preferredthat the substrate thickness be from about 30 to about 85% of theoverall structure. This, in combination with the population andconfiguration of the voids in a structure at least about 0.5 mils thick,will materially contribute to the overall degree of opacity of thestructure. Likewise, by maintaining the barrier layer thickness within aparticular range in relation to the overall structure and to thethickness of the substrate layer, the overall combination results in alustrous satin appearance. It is preferred that the barrier layerthickness (or combined barrier layer thickness where such a layer isaffixed to both surfaces of the substrate layer) be about 15 to about70% of the overall film structure. It is important that the barrierlayer be sufficiently thick so that the outer surface thereof does notmanifest the irregularities or surface projections of the substratelayer material. If this were not the case, the lustrous appearance ofthe satin finish would be materially lessened.

When preformed spheres are employed, it is the shape and size of thesphere that is important rather than the chemical nature of thematerial, per se. Thus, solid or hollow organic or inorganic spheres ofany type can be employed. Interesting effects can be achieved by the useof spheres of different colors. Since statistically each void hasapproximately one sphere somewhere within the void, interesting andpleasing color and/or reflectance effects can be imparted to the overalllayer structure by the use of spheres of different color absorption orreflectance. The light scattered in a particular void is additionallyeither absorbed or reflected by the void initiating sphere and aseparate color contribution is made to the light scattering in eachvoid.

Examples of thermoplastic resins which can be used as the dispersedphase within the matrix material are the polyamides or nylons ofcommerce, certain polyesters such as polyethylene terephthalate acetals,acrylic resins, etc. Inorganic materials which are useful as voidinitiating particles include solid or hollow preformed glass spheres,metal beads or spheres, ceramic spheres, etc. In fact, any materialwhich can be formed into spheres without causing thermal degradation tothe substrate material is contemplated.

By the technique of the present invention, light transmission throughthe laminate of this invention can be reduced to as low as about 16%.This would be true in a film having an overall thickness of at least 1.5mils where the substrate layer is at least 60% and the barrier layer(s)is 20%.

While the preferred particle size of the sphere is from about 0.1 toabout 10 microns, it is particularly preferred that the particle sizerange from about 0.75 to about 2 microns. The void initiating particlescan be present in up to about 20% by weight of substrate layer (a) priorto orientation, a preferred range being from 2 to about 7 percent byweight.

For convenience and more precise control of the formulation andcharacter of the substrate layer, a master batch technique can beemployed either in the case of forming the spherical particles in situor in adding preformed spheres to the molten thermoplastic matrixmaterial from which substrate layer (a) will be formed. Afterpreparation of a master batch, appropriate dilution of this system canbe accomplished by adding additional thermoplastic matrix material untilthe desired proportions are obtained.

It is preferred that the substrate material and the barrier material becoextruded. Thereafter, biaxial orientation is carried out to an extentand at a temperature calculated to obtain the maximum degree of opacitywithout any significant sacrifice in the physical characteristics,including appearance, of the resulting laminate film structure.Obviously, as the materials employed change, the conditions of biaxialorientation will change. By way of exemplification when employingnylon-6 as the void initiating spheres, a machine direction orientationof from 4 to 8 times and a transverse direction orientation of fromabout 4 to 8 times, at a drawing temperature of from 100° C. to 160° C.to yield a biaxially oriented film of from 0.7 to 3 mils of overallthickness can be obtained.

The thermoplastic water vapor transmission barrier layer (b) can befabricated from a wide variety of resins, the poly-alpha-olefins such aspolyethylene, polypropylene, polyethylene-polypropylene copolymer, andthe like, being preferred. Of these, isotactic polypropylene isespecially preferred. Thickness of this layer can vary, e.g., from about2 to 10 microns, and is preferably in the range of about 4 to 6 microns.Barrier layer (b) contains a hygroscopic particulate or microfibrousmaterial in an amount sufficient to impart effective water vaporabsorbent capability to the resin. Thus, for example, from about 5 to 40weight parts, and preferably from about 10 to 30 weight parts, ofhygroscopic material having an average diameter of from about 0.5 to 5microns or so (depending on the thickness of layer (b)) can be uniformlyincorporated into the resin comprising layer (b) prior to its extrusion.If desired, these materials can be pretreated with chemical couplingagents, e.g., silane or titanate type coupling agents, in a known mannerto enhance the adhesion of the materials to the resin in which they areincorporated. Suitable hygroscopic materials include any of a largevariety of organic and inorganic substances which are known to absorbwater vapor due to physical and/or chemical affinity for water, e.g.,diatomaceous earth, dehydrated inorganic salts and hydroxides, driedhighly pulverized clays, talcs, silica gels, water-absorbentcellulosics, nylon-type polymers, ethylene-vinyl alcohol copolymer,ethylene-vinyl acetate-vinyl alcohol terpolymer having a relatively highvinyl alcohol content, and so forth. Alternatively, barrier layer(b) maybe fabricated entirely from a hygroscopic thermoplastic film formingresin such as the aforementioned ethylene-vinyl alcohol copolymer andethylene-vinyl acetate-vinyl alcohol terpolymer. Optionally, barrierlayer (b) can contain an opacifying pigment in conventional amountsuniformly distributed therein.

In order to enhance the cohesiveness and adherence of substrate layer(a) to barrier layer (b), known and conventional adhesive resins can beemployed. For example, the CXA resins available from DuPont (Wilmington,Del.), a series of coextrudable adhesive resins for multilayer resinfilms, can be advantageously employed for this purpose.

It is also within the scope of the present invention to prove substratelayer (a) with a transparent and/or pigmented skin layer (c) as is knownfrom U.S. Pat. No. 4,377,616, supra. In general, such a layer will bemuch like that of barrier layer (b) excpet for lacking a hygroscopiccomponent or capability.

In keeping with the invention, a number of different laminatearrangements can be provided as illustrated in the following table.

                                      TABLE                                       __________________________________________________________________________    VARIOUS LAMINATE STRUCTURES                                                   LAMINATE                                                                             LAYER 1                                                                              LAYER 2                                                                             LAYER 3                                                                              LAYER 4                                                                             LAYER 5                                                                              LAYER 6                                                                             LAYER 7                         __________________________________________________________________________    1      substrate                                                                            barrier                                                                              --     --    --     --    --                                    layer (a)                                                                            layer (b)                                                       2      barrier                                                                              substrate                                                                           barrier                                                                               --    --     --    --                                    layer (b.sub.1)                                                                      layer (a)                                                                           layer (b.sub.2)                                           3      transparent                                                                          substrate                                                                           barrier                                                                               --    --     --    --                                    or pigmented                                                                         layer (a)                                                                           layer (b)                                                        layer (c)                                                              4      transparent                                                                          substrate                                                                           barier substrate                                                                           barrier                                                                               --    --                                    or pigmented                                                                         layer (a.sub.1)                                                                     layer (b.sub.1)                                                                      layer (a.sub.2)                                                                     layer (b.sub.2)                                                                       --    --                                    skin layer                                                                    (c)                                                                    5      transparent                                                                          substrate                                                                           barrier                                                                              substrate                                                                           transparent                                                                           --    --                                    or pigmented                                                                         layer (a.sub.1)                                                                     layer (b)                                                                            layer (a.sub.2)                                                                     or pigmented                                        skin layer                skin layer                                          (c.sub.1)                 (c.sub.2)                                    6      barrier                                                                              substrate                                                                           barrierr                                                                             substrate                                                                           barrier                                                                               --    --                                    layer (b.sub.1)                                                                      layer (a.sub.1)                                                                     layer (b.sub.2)                                                                      layer (a.sub.2)                                                                     layer (b.sub.3)                              7      transparent                                                                          substrate                                                                           transparent                                                                          barrier                                                                             transparent                                                                          substrate                                                                           transparent                            or pigmented                                                                         layer (a.sub.1)                                                                     or pigmented                                                                         layer (b)                                                                           or pigmented                                                                         layer (a.sub.2)                                                                     or pigment-                            skin layer   skin layer   skin layer   ed skin                                (c.sub.1)    (c.sub.2)    (c.sub.3)    layer (c.sub.4)                 __________________________________________________________________________

The following examples are illustrative of polymer laminates of thepresent invention.

EXAMPLE 1

This example provides Laminate No. 2 of the above Table, i.e., athree-layer laminate in which substrate layer (a) is interposed betweentwo water vapor transmission barrier layers (b₁) and (b₂).

A mixture of isotactic polypropylene (93 parts, MP 160° C. and a meltindex of 4.5) and nylon-6 (7 parts, MP 225° C.) from which substratelayer (a) is to be formed is melted in an extruder provided with a screwof L/D ratio of 20/1. A second extruder in association with this firstmentioned extruder is supplied with the same polypropylene without thenylon-6 present but filled with 20 weight parts of uniformly distributeddiatomaceous earth particles (average particle size of about 2 microns)and serving as the resin from which barrier layers (b₁) and (b₂) are tobe formed. A melt coextrusion is carried out while maintaining thecylinder of substrate material (a) at a temperature ranging from 190° to220° C. The polypropylene to be extruded as barrier layers (b₁) and (b₂)is maintained at a temperature of from 190° to 220° C. A film structureis coextruded with a substrate thickness 40 % of the total extrudedthickness. The barrier layers are approximately 30% of the totalthickness. The unoriented film measures approximately 40 mils inthickness. This sheet is subsequently oriented seven by seven andone-half times using a commercially available sequential biaxiallyorienting apparatus. The MD orientation temperature is about 105° C. andthe TD orientation 135° C. The resulting 1.9 mil film has an opacity of20% transmission and a 45° gloss of 120%.

EXAMPLE 2

This example provides Laminate No. 7 of the above Table, i.e., aseven-layer laminate in which a barrier layer (b) is interposed betweentwo three-layer laminate structures in which both surfaces of asubstrate layer are affixed to transparent skin layers as in U.S. Pat.No. 4,377,616.

The coextrusion procedure of Example 1 is followed except that thesecond extruder is supplied with uncompounded polypropylene (at anextruder temperature of about 220° C.) from which each of twotransparent skin layers will be formed and a third extruder is arrangedto provide the barrier layer. Melt coextrusion of the three polymerstreams is carried out to form the foregoing seven-layer laminate ofabout 40 mils thickness. In the resulting film structure, opacifyingsubstrate layers (a₁) and (a₂) together represent about 50% of the totalextruded thickness, skin layers (c₁) to (c₄) make up 30% of thisthickness and barrier layer (b) constitutes the remaining 20% of thelaminate thickness.

EXAMPLE 3

The same procedure as in Example 1 is followed but to providefive-layered Laminate No. 4 of the above Table. Omitting transparentskin layers (c₂) and (c₃) of Laminate No. 7 puts all of the skin resinon the outer layers of the structure thereby enhancing the stiffness ofthe laminate.

EXAMPLE 4

Following substantially the same procedures as previously described,five layered Laminate No. 6 is provided. In addition, each of thebarrier layers is provided with about 5% by weight of titanium dioxidepigment to further augment opacity. The resulting arrangement of threealternating barrier layers (b₁) to (b₃) presents multi-stage water vaportransmission barrier capability for even more effective barrierproperties.

EXAMPLE 5

Example 3 is substantially repeated except that skin layers (c₂) and(c₃) are loaded with about 5% by weight of sodium carbonate to enhanceopacity. The exposed surface of skin layer (c₂) is corona treated in aknown and conventional manner to improve its printability.

What is claimed is:
 1. An opaque barrier layer which comprises:(a) atleast one thermoplastic polymer matrix substrate layer within which islocated a strata of voids;positioned at least substantially within atleast a substantial number of said voids is at least one sphericalvoid-initiating particle which is phase distinct and incompatible withthe matrix material, the void space occupied by said particle beingsubstantially less than the volume of said void, with one generallycross-sectional dimension of said particle at least approximating acorresponding cross-sectional dimension of said void; the population ofvoids in said substrate and the thickness of said substrate being suchas to cause a degree of opacity of less than 70% light transmission;and, (b) at least one thermoplastic water vapor transmission barrierlayer affixed to said thermoplastic substrate layer.
 2. The polymerlaminate of claim 1 wherein in said substrate layer (a), the sphericalparticles are an inorganic material.
 3. The polymer laminate of claim 1wherein in said substrate layer (a), the spherical particles are anorganic material.
 4. The polymer laminate of claim 3 wherein saidorganic material is a polymer.
 5. The polymer laminate of claim 1wherein said substrate layer (a) is coextruded with said barrier layer(b).
 6. The polymer laminate of claim 1 wherein the substrate thicknessis from about 30 to about 85% of said structure.
 7. The polymer laminateof claim 1 wherein said barrier layer (b) is of the same polymericmaterial as said substrate material (a).
 8. The polymer laminate ofclaim 1 further possessing at least one skin layer (c) affixed tosubstrate layer (a).
 9. The polymer laminate of claim 8 whereinsubstrate layer(s) (a), barrier layer(s) (b) and skin layer(s) (c) arethe same polymeric material.
 10. The polymer laminate of claim 1 whereinbarrier layer (b) contains from about 5 to about 40 weight parts ofhygroscopic material uniformly distributed therein.
 11. The polymerlaminate of claim 10 wherein barrier layer (b) contains from about 10 toabout 30 weight parts of hygroscopic material uniformly distributedtherein.
 12. The polymer laminate of claim 10 wherein the hygroscopicmaterial is diatomaceous earth, dehydrated salt or hydroxide, driedhighly pulverized clay, talc, silica gel, water absorbent cellulosic,water absorbent polyamide, ethylene-vinyl alcohol copolymer orethylene-vinyl acetate-vinyl alcohol terpolymer.
 13. The polymerlaminate of claim 1 wherein barrier layer (b) is fabricated from a waterabsorbent polymer containing 0 to 40 weight parts of hygroscopicmaterial uniformly distributed therein.
 14. The polymer laminate ofclaim 13 wherein barrier layer (b) is fabricated from ethylene-vinylalcohol copolymer.